1. Field of the Invention
This invention relates to a tilt angle measuring apparatus that is mounted on a bottom surface of a vehicle, for example, and measures a tilt angle of the vehicle with respect to a road by a transmitted wave which is emitted toward a road surface and a received wave which is reflected by the road surface.
2. Description of the Related Art
This kind of conventional tilt angle measuring apparatus is provided with an ultrasonic wave generating device for generating an ultrasonic wave, a pair of front and rear sonars that emit the ultrasonic waves which are generated by the ultrasonic wave generating device toward a driving road, and detect a signal reflected by the driving road, and a reflection signal processing device that determines distances between two sonars on a vehicle and the driving road from the signals detected by these sonars (for example, see a patent document 1: Japanese Unexamined Patent Publication No. 56-8727).
On the other hand, an ultrasonic wave transmitting/receiving device provided with a pair of ultrasonic vibrators is disclosed. In this ultrasonic wave transmitting/receiving device, one ultrasonic vibrator is used for emitting the ultrasonic wave and the other ultrasonic vibrator is used for receiving the ultrasonic wave. In both of the ultrasonic vibrators, a pair of horns of the same shape are arranged in parallel and integrally formed of resin. Each of the horns has a diameter which gradually increases from a throat portion to an opening portion and both of the horns are integrally connected to each other in such a way that each opening portion opens at a baffle portion that is a nearly rectangular plate. Then, a baffle face of an outer surface of the baffle portion is divided by concave grooves or convex walls thereby to reduce the ultrasonic wave emitted from one ultrasonic vibrator toward an object being diffracted and received by the other ultrasonic vibrator (for example, see a patent document 2: Japanese Unexamined Utility Model Publication No. 55-144481).
As is understood from a rectangular side face of the sonar, it is thought that wave energy of a signal reflected by the driving road becomes weak because the ultrasonic wave emitted by the sonar is diffused in a wide range. In order to solve this weak wave energy problem, it is necessary to electrically amplify the weak ultrasonic wave detected by the sonar. However, amplifying the weak ultrasonic wave inevitably amplifies ambient noises at the same time, so that it is thought difficult to discriminatively detect the ultrasonic wave which is desired to detect from the ambient noises (for example, see the patent document 1).
On the other hand, in the ultrasonic wave transmitting/receiving device, the pair of horns of the same shape of both of the ultrasonic vibrators, are arranged in parallel and integrally formed of resin. Thus, in an occasion where the horn is damaged, the whole horns including both of the ultrasonic vibrators need to be replaced. Moreover, an inclined face that is an inner surface of the horn is connected to the baffle face that is an outer surface of the baffle portion, by one bending portion. Thus, a sufficiently large amount of wave energy exists near the bending portion and a part of the whole wave energy but not a little wave energy, is diffused in a lateral direction along the baffle face. In other words, it is thought that an amount of wave energy diffracted in a direction parallel to the baffle face, that is, in a direction lateral to a direction in which the wave energy travels, becomes large by diffraction phenomenon and that the wave energy cannot be concentrated in a target direction (for example, see patent document 2).
At the same time, since the ultrasonic wave transmitting/receiving faces of the sonars are bared at the case, it is thought that bounced stones directly hit the ultrasonic wave transmitting/receiving faces and splashed water enters into the sonars (for example, see patent document 1). In addition, it is also possible to come up with an idea of the case provided with the horns, but in this occasion it is thought that the horns are damaged by bounced stones (for example, see patent document 1 and patent document 2).